Discrete neural networks within the central nervous system of both vertebrates and invertebrates are responsible for generating the patterned neural activity that mediates rhythmic behaviors, such as locomotion, respiration and the chewing and processing of food. Each network produces multiple distinct but related motor patterns as a result of modulatory inputs from other regions of the nervous system. A long-term goal of this proposal is to understand the cellular mechanisms underlying the selection and generation of the appropriate motor pattern at the behavioral appropriate time. This includes identifying the projection neurons responsible for activating, modulating and terminating neural network activity, and characterizing the effects of each projection neuron on the neural network and on other projection neurons to that network. This also includes studying the presence and functional consequences of presynaptic inputs onto the spatially and electronically distant terminals of these projection neurons. This proposal focuses on modulation of the well-characterized pyloric and gastric mill motor patterns in the crustacean stomatogastric ganglion (STG) by distinct projection neurons that innervate the STG, in the crab Cancer borealis. Electrophysiological techniques, including simultaneous intrasomatic recordings of STG network neurons and both intrasomatic and intra-axonal recordings of projection neurons, as well as dye filling and double labeling techniques will be used to address the following hypotheses: (1) Different projection neurons use different strategies to elicit distinct pyloric and gastric mill motor patterns. (2) Motor pattern selection in the STG by projection neurons includes presynaptic influences onto the STG terminals of other projection neurons. (3) There is an extensive network of electrical coupling in the STG involving the terminals of individual projection neurons. (4) Presynaptic input onto a projection neuron functionally compartmentalizes the activity of that neuron within the STG neuropil. The results of these experiments will provide a new level of understanding, previously unobtainable, about how individual projection neurons influence neural network activity, including the roles played by presynaptic inputs onto the distant terminals of these projection neurons. This will help guide conceptual understanding and experimental approaches aimed at understanding neural network modulation in the similar, but less accessible, vertebrate nervous system.